This project studies the effect of behavioral, environmental and pharmacological variables on responses of thalamic projection neurons in the medullary dorsal horn (trigeminal nucleus caudalis) to noxious and innocuous thermal stimuli. Rhesus monkeys are trained to detect the termination of innocuous heat stimuli and the onset of noxious heat stimuli. In a second task, these monkeys detect the onset of a light stimulus. Trigeminothalamic neurons code thermal discriminative information used by the monkey within the behavioral tasks. The thermal sensitivity of many of these neurons vary with the behavioral significance of the stimulus. In addition, some neurons that respond to passive mechanical and thermal stimulation also respond to other stimuli the monkey uses for successful completion of the task. This task-related activity occurs in several patterns of excitation and inhibition and some neurons showing each pattern project to the thalamus. Task-related responses may modulate sensory activity and thereby influence the perception of and response to oral-facial pain. Preliminary data suggest that local application of morphine selectively attenuates activity of nociceptive neurons and affects the monkeys' ability to discriminate noxious from innocuous temperatures.